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ABSTRACT

1. A METHOD OF DETECTING AND MONITORING THE PRESENCE OF AN ELECTRON CAPTURE HALOGENATED TRACER GAS IN A GAS SUPPLY HAVING AS A CONSTITUENT THEREOF AN ELECTRON CAPTURE MATERIAL, COMPRISING THE STEPS OF SELECTIVELY ISOLATING SAID HALOGENATED TRACER GAS FROM THE REMAINDER OF THE SUPPLY THE DIFFUSION THROUGH A MEMBRANCE FORMED FROM A SILASTOMER HAVING A GREATER PERMEABILITY OF SAID HALOGENATED TRACER GAS THAN TO THE REMAINDER OF THE SUPPLY, AND THEREAFTER CONVEYING THE TRACER GAS INTO AN ELECTRON CAPTURE DETECTOR BY MEANS OF A GAS STREAM SUBSTANTIALLY INERT TO ELECTRON CAPTURE.

May 6, `975 A JENKINS E'rAL Re. 28,417

METHOD AND APPARATUS FOR DETEGTING TRACER GAS Original Filed Aug. 15,1970 United States Patent Re. 28,417 Reissued May 6, 1975 28,417 METHODAND APPARATUS FOR DETECTING TRACER GAS Anthony Jenkins and Richard C.Cornell, Royston, England, assignors to Analytical Instruments, Ltd.,Royston, England Original No. 3,699,342, dated Oct. 17, 1972, Ser. No.63,571, Aug. 13, 1970. Application for reissue May 21, 1973, Ser. No.362,432 Claims priority, application Gigas Britain, Apr. 27, 1970,

1 Int. Cl. G01c 1/18 U.S. Cl. Z50-302 5 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue specilicatlon; matter printed in italica indicates'the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE A method and apparatus for detecting thepresence of an halogenated tracer gas, which is an electron absorber. inan environment which is itself, or comprises as a constituent, anelectron absorber. The apparatus comprises an electron capture detectorand the environment to be sampled is rst drawn through a membrane whichhas a greater permeability to the tracer gas than the remainder of theenvironment before being carried into the detector by a carrier gas.This results in a diminution of the concentration of any electroncapture agents present in the environment and which if drawn into thedetector could swamp the effect of the tracer gas.

The present invention relates to a method and apparatus for detectingthe presence of electron capture tracer gases. As examples of such gasesthere can be mentioned SP3, CZHZCIZ, C2H3BI'4, C2H2Cl4, CClgFg andCBI'CIFZ. Such tracer gases are electron absorbers and the apparatus isparticularly concerned with the detection of such tracer gases presentin an environment which is itself, or comprises as a constituent, anelectron absorber.

One use for tracer gases is the detection of leaks in a gas system. Intwo known methods the tracer gas has been helium, which is detectable bya mass spectrometer and freon which is detectable by a halogen detector.The mass spectrometer is both bulky and expensive. The halogen detectorsare not suciently sensitive for a number of applications.

The present invention utilizes an electron capture detector fordetecting the presence of the tracer gas. A11 electron capture detectorgenerally comprises an ionization chamber containing a particle emitter,such as tritium, as the primary source of ionizing radiation. Upon entryinto the chamber of a carrier gas such as nitrogen possessing noaffinity for electrons, recombination of positive ions nad freeelectrons formed by the ionizing radiation is unlikely to take placebecause of the free electrons high mobility. Thus by applying a smallpotential across the chamber all ions formed by the ionizing radiationcan be collected. When the carrier gas contains a compound having anaflinity for electrons, negative ion formation occurs which isaccompanied by an observed decrease in current.

The present invention seeks to provide a method and apparatus which canbe used to monitor atmospheric air or other gas samples for the presenceand concentration of sulphur hexauoride and other halogenated tracergases. Oxygen itself is capable of electron capture and it couldtherefore saturate the detector.

According to one aspect of the present invention a method of detectingand monitoring the presence of ha1ogenated tracer gas in a gas supplyhaving as a major constituent thereof an electron capture materialcomprises selectively isolating said tracer gas from the remainder ofthe supply by diifusion through a membrane having a greater permeabilityto said tracer gas than to the remainder of the supply, and thereafterconveying the tracer gas into an electron capture detector by means of agas stream substantially inert to electron capture.

According to another aspect of the present invention, an apparatus fordetecting and monitoring the presence of halogenated tracer gas supplycomprises a probe carrying a membrane having a greater permeability tosaid halogenated tracer gas or gases than the remainder of the gassupply, means for conveying a carrier .gas to the side of the membraneremote from the gas supply and an electron capture detector to receivesaid carrier gas containing the halogenated tracer gas whereby to detectthe latter.

When the halogenated tracer gas is present in the atmosphere it isnecessary to separate the tracer gas from the oxygen of the atmosphereprior to introduction into the detector. Oxygen is itself an electronabsorber and its presence would saturate the detector and effectivelyblanket the presence of the tracer gas. Suflicient amount of the tracergas must be introduced intothe detector to obtain a useful response ofthe detector and it is therefore necessary to separate the tracer gasfrom the atmospheric oxygen prior to its introduction into the detector.

This is achieved by separating the detector from the atmosphere or othergas supply containing a preponderance of an electron capture material inaddition to the electron capture tracer gas by means of a membrane. Thetracer gas can pass through the membrane and is then swept into thedetector by a stream of inert gas i.e. inert to electron capture. Asuitable carrier gas is nitrogen.

A suitable membrane is formed from a silastomer.

The invention will be described further by way of example with referenceto the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of an apparatus according to theinvention, land FIG. 2 shows a modification of part of the apparatus inwhich a sampling probe is contained in an outer sheath.

A sampling probe 10 for introducing a gas to be analyzed into anelectron capture detector is closed at one end by a membrane 12. Themembrane is preferably formed from a silastomer as this has apermeability to some halogenated tracer gases which is of the order often times .greater than that to oxygen. The opposite end of the probe isconnected through a line 14 controlled by a valve 16 to a nitrogensupply, the nitrogen serving as a carrier gas to convey the gas to beanalyzed into the electron capture detector 18 by way of a line 20. Theline 20 commences adjacent the membrane 12. An amplifier 22 and a meter24 are connected to the detector. When the gas to be analyzed containstracer gas and is introduced into the detector this results in a changein the ionization current which charge is indicative of theconcentration of tracer gas in the nitrogen carrier gas stream and isnoted on the meter 24.

The term silastomer refers to a thin lm or sheet of silicone rubber.

In FIG. 2, the sampling probe 10 is contained within an outer sheath 30.The end of the sheath 30 adjacent the membrane 12 is open to theatmosphere while the other end of the sheath commnuicates through line32 with a suction means, for example an air pump (not shown). The pumpdraws air or gas to be sampled into the sheath and into contact with themembrane Furthermore, as shown in FIG. 2, the exhaust from the detector18 can be connected to the suction line 32. This provides a convenientmethod of determining ow from the detector. The draft created by thesuction makes the probe less directionally sensitive and it can pick upa tracer gas at greater distances from a leak than th apparatus ofFIG.*1.

The apparatus can be used for detecting leaks in a gas system having ahalogenated tracer gas. Examples of tracer agents are sulphurhexauorideSF and Freon (registered trademark) such asdichlorodiuoromethane 1. A method of detecting and monitoring thepresenceof an electron capture halogenated tracer gas in a gas supplyhaving as va constituent thereof an `electron capture material,comprising the steps of selectively isolating said halogenated tracergas from the remainder of thel supply by diiusion through a membraneformed from a silastomer having a greaterpermeability to'said halogenated tracer gas than to the remainder of the supply,

and thereafter conveying the tracer gas intoan electron capture detectorby means of a gas stream substantially inert to electron capture.

2. An appparatus for detecting and monitoring the pres-` ence of anelectron capture halogenated tracer gas in a gas supply which is itself,or includes as a constituent, an electron capture material, theapparatus comprising a probe, a membrane at an end of said probe inwhich the membrane comprises a silastomer having a greater permeabilityto the halogenated tracer gas than the remainder of the gas supply,means for conveying a carrier gas to the side of the membrane remotefrom the gas supply and an electron capture detector to receive thecarrier gas and tracer gas.

3. An apparatus according to claim 2, including an outer.-sheathsurrounding thetprobe andeopen to theatmosphere at one end,suction means being connected to the other end of the sheath whereby todraw in a sample into the vicinity of the membrane.

4. A method of detecting and monitoring the presence of an electroncapture nitro-compound tracer gas in a gas supply having as aconstituent thereog an electron capture material, comprising the stepsof selectively isolating said nitro-compound tracer gas from theremainder of the supply by diffusion through a membrane formed from asilastomer having agreater permeability to said tracer gas than to theremainder of the supply, and thereafter' conveying the tracer gas intoan electron capture detector by means of a gas stream substantiallyinert to electron capture. v

45. An apparatus for detecting and monitoring the presence of 1anelectron ,capture nitro-compound tracer gas in a gas supply which isitself,A or includes as a constituent,

an electron capture material, the apparatus comprising a probe, amembrane at an end of said probe in which the` membrane comprises asil'astomer having a greater permeability `to the nitro-compound tracergais` than the regmainder of the gas supply, means for conveying acarrier gas' to the side 'of the membrane remote from the gas supply andan Aelectron capture detector to receive the carrier gas and tracer gas.

References Cited i The followingl references,'cited by the Examiner, areof record in the patented le of this patent or the original patent.

UNITED STATES PATENTS 1/1968 Petitjean et al Z50-381 X 12/1970 McKinley,Jr. 55-16 ARCHIE R. BORCHELT, Primary Examiner l U.S. Cl. X.R. 250, 389

